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This question already has an answer here:

I have been following the developments around the Shawyer EM drive for about 2 years now as NASA and other parties test it out in various environments. To preface, I would very much like to see the EM drive work so sorry in advance if I am afflicted with a case of confirmation bias.

The most recent test in Dresden, Germany (the paper by M. Tajmar et al.) to me seems like another test that proves this idea may have some validity. As you may know if you are thinking about answering this question, the main concern with this idea is that it violates the momentum conservation principle (i.e., there must be something else that is causing the "thrust" force the experimenters are observing).

At this point, I need to say my background in physics is only through quantum mechanics and special relativity. I have dabbled in General Relativity and Quantum Field Theory, but don't yet have the mathematical prowess to fully appreciate them. I am an engineer, not a physicist, so forgive me if my arguments are a bit flawed. That's why I'm asking after all!

From thermodynamics, we know energy must be conserved (energy conservation). Conventional wisdom would say that momentum must also be conserved; however, I have begun to think this is solely a classical dynamics idea. I know in quantum mechanics, the transfer of momentum is also conserved, but when I try to correlate this with General Relativity's view of the world in terms of the stress-energy tensor, I find that momentum flux (and thus in my mind, momentum) is merely a product of an energy density. After all, if mass is just energy in the form of mass-energy, why can't this mass-energy move thus making momentum? It would seem to me that the so-called "conservation of momentum" is just another way of saying "energy is conserved" in a different way.

In the Shawyer drive, energy is conserved: you still have to provide energy to the system, and in return you get thrust. If momentum is truly just another form of energy, why do skeptics think this can't work?

So to be succinct (sorry, that was long-winded), why is the momentum conservation law so important if everything boils down to energy at the end of the day? If everything (and I mean everything) truly is just different forms of energy, shouldn't it be obvious that it must be possible to convert it from one form to another via some pathway or process?

Thanks in advance for helping me with my little thought!

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marked as duplicate by Emilio Pisanty, stafusa, Kyle Kanos, John Rennie, Jon Custer Nov 15 '17 at 16:41

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ The EMDrive is bunk, read any competent news article on the matter, which has been done to death on this site. Momentum conservation is a result of a Noetherian symmetry (spatial invariance) and can't be dismissed because you want it to. $\endgroup$ – Kyle Kanos Jul 28 '15 at 21:18
  • $\begingroup$ How do we know "from thermodynamics" that energy is conserved? Energy and momentum are conserved because of time and space translation invariance of the physical laws. $\endgroup$ – ACuriousMind Jul 28 '15 at 21:18
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    $\begingroup$ Note also that Tajmer's "paper" is a not-yet published conference proceeding, which is not peer reviewed. $\endgroup$ – Kyle Kanos Jul 28 '15 at 21:20
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    $\begingroup$ There was some discussion of this matter in the h bar (our main chatroom) earlier today. Be aware that it is interspersed with some non-physics content in the way such things often are at the local watering hole. $\endgroup$ – dmckee Jul 28 '15 at 21:31
  • $\begingroup$ Thanks for the responses guys. I have a few follow-up questions if you have time. I read up on the Noetherian Symmetry (thanks Kyle). From conventional wisdom (and my dynamics courses), I am inclined to agree; but I would like to press further. Physically speaking, can the conservation of momentum be traced back to the conservation of energy? Or am I totally wrong in that regard? $\endgroup$ – hakzatchel Jul 29 '15 at 6:43
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You can read the abstract of the Tajmar paper here: http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083. Important quote:

We identified the magnetic interaction of the power feeding lines going to and from the liquid metal contacts as the most important possible side-effect that is not fully characterized yet. Our test campaign can not confirm or refute the claims of the EMDrive but intends to independently assess possible side-effects in the measurements methods used so far. Nevertheless, we do observe thrusts close to the actual predictions after eliminating many possible error sources that should warrant further investigation in the phenomena.

No one is claiming to have irrefutably demonstrated a working device. The resultant thrusts are tiny--$20\ \mu{}N$ as compared to $20\ mN$ from ion rockets for similar power input--and experiments are still in the debugging phase to make sure that later measurements of this tiny effect are not swamped by various sources of noise and error.

As for why most physicists are skeptical, there has never been an experiment that demonstrated momentum not being conserved. Every time an experiment seemed to show this, a new interaction was discovered that restored momentum to a constant value. A classic example of this is the discovery of the neutrino, when the products of nuclear decay seemed to gain momentum from nowhere. The solution turned out to be a particle that escaped from the experiment undetected.

Nobody should be discouraged from following the news of this device nor performing experimental work. However, recent press releases have badly mischaracterized the actual work being performed with this device.

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  • $\begingroup$ Thanks for the responses Mark. I agree, the experiment does seem to have lots of possible sources of error, and as I wrote back to the comments on my question I am willing to get on board with the whole momentum conservation law. As you say, the device may be creating a particle that is undetectable with the sensors they have (if they are looking for it at all). If particles are created that have a momentum, then obviously there must be a reaction force. Is this is impossible in the physics of the EM Drive, and if so why? I am not qualified to make a claim on this matter, but I'd like to know. $\endgroup$ – hakzatchel Jul 29 '15 at 6:53
  • $\begingroup$ @hakzatchel In principle, nothing is 100%, for certain impossible, but past experience gives scientists good instincts about which proposals to take seriously. We have been studying microwave and RF cavities for decades and have many tools to analyze them (I work for a company that builds particle accelerators, so I'm surrounded by these things). For such a simple device (not too dissimilar from a microwave oven) to have such unexpected properties invites plenty of skepticism. $\endgroup$ – Mark H Jul 29 '15 at 10:01
  • $\begingroup$ If this thing works as described by Shawyer, no arguments or advocacy will be necessary. There will be a scramble to build these things and the first person to give a rigorous explanation for how they work is a shoo-in for the Nobel Prize. I say rigorous because, as yet, all proposed explanations of how these devices are supposed to work are met with "That's not how electromagnetic fields/virtual particles/relatvity/quantum mechanics works." $\endgroup$ – Mark H Jul 29 '15 at 10:10
  • $\begingroup$ You make good points; I can see why skepticism is warranted for this device. Thanks for helping! I think I will still need to dig deeper into the physics of this problem though (I'm still unsatisfied because I don't fully comprehend why the EM Drive does, or doesn't work), but I will keep these ideas in mind as I do so. $\endgroup$ – hakzatchel Jul 29 '15 at 10:13
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The EM drive is in the headlines again on Yahoo.com.

In a new round of testing, NASA confirms yet again that the 'impossible' EMdrive thruster works

There exists a web page and a "theory" by the proponent

Suppose that an effect really exists. The main objection is momentum conservation, because the setup sounds like a variant of the bootstrap effect: a drowning man cannot pull up himself by his boodstraps.

In the site there is a FAQ the answer an the momentum conservation problem :

Q. Why does the EmDrive not contravene the conservation of momentum when it operates in free space?

A. The EmDrive cannot violate the conservation of momentum. The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection. The momentum gained by the EmDrive plus the momentum lost by the electromagnetic wave equals zero. The direction and acceleration that is measured, when the EmDrive is tested on a dynamic test rig, comply with Newtons laws and confirm that the law of conservation of momentum is satisfied.

This answer may suffice for the lab, but it is ignoring that the "electromagnetic wave momentum is built up in the resonating cavity" and could be balanced by the fixed position of the apparatus. (When running, the forward momentum is balanced by the opposite momentum of the earth).

EMdrive

In a space environment the whole apparatus has to balance the momentum. If there is thrust, (i.e. the effect does not disappear) the only explanation, imo, would be electromagnetic radiation (photons) leaving the apparatus with momentum equal and opposite to the forward momentum. It is a metallic apparatus and could very well radiate like a directional antenna . This could be checked under lab conditions.

As to why momentum has to be conserved? It is because it is one of the laws of nature that has never been broken. The neutrino was discovered by imposing momentum conservation. A breaking would hit the foundation stones of present day theories.

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